They're not three independent formats
FIT is what your device actually records. GPX and TCX are both generated from that FIT file when you export or share an activity — they're lossy exports of the same underlying recording, not separate data sources. Picture it like a funnel:
FIT (GPS, HR, power, cadence, laps, device metadata, running dynamics — everything) ├── TCX (GPS + heart rate + cadence + power) └── GPX (GPS + heart rate + cadence; usually no power)
Each step down that funnel keeps less. Once something's dropped in the export, it's gone — a GPX file was never going to have power data, no matter how good your device's recording was.
GPX
GPX (GPS Exchange Format) is the most universal of the three — it's a simple XML schema built around a track of timestamped GPS points (latitude, longitude, elevation). Heart rate and cadence can ride along as vendor extensions, and most exporters include them, but power, running dynamics, temperature, and device-specific fields generally don't have anywhere to go in GPX's schema. It's the safest choice if you just need a route to plot on a map or import into another mapping tool — and the weakest choice if you want to analyze effort or physiology.
TCX
TCX (Training Center XML) is Garmin's own format, and it sits between GPX and FIT: still XML, still human-inspectable, but built with training data in mind from the start. It typically carries GPS, heart rate, cadence, and — for devices that record it — power, organized into laps and trackpoints rather than one flat stream. It's a reasonable middle ground if you want more than GPX gives you but don't need every last field FIT has.
FIT
FIT is the source, not an export — see our full breakdown of what's inside a FIT file for the details. Short version: it's binary (not human-readable without decoding), and it holds everything your device actually recorded — running dynamics, device settings, event markers, and whatever else your specific watch or bike computer captures that no export schema anticipated.
Which one should you use?
If you already only have a GPX or TCX file — from an old export, a third-party app, or a device that never wrote FIT — that's what you have, and it's still useful. But if you have a choice, and you care about getting the most detail into an AI chat or any other analysis, always reach for the FIT file. Garmin Connect and Strava both let you export the original FIT alongside (or instead of) GPX/TCX — look for an "export original" or "export file" option rather than the default GPX download.
This tool handles all three
Whichever format your export gives you — FIT, GPX, or TCX — this tool decodes its full native structure into JSON with nothing dropped, then lets you combine exactly the date range you want into one Markdown file sized to paste into ChatGPT, Claude, or any other chatbot. Everything runs in your browser; nothing is uploaded.